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  • The elephant is a creature of epic proportions,

    大象是體積超龐大的生物

  • and yet it owes its enormity to more than 1,000 trillion microscopic cells,

    這麼碩大的體積,是由超過一千兆個微小的細胞所構成

  • and on the epically small end of things,

    而在微觀世界中

  • there are likely millions of unicellular species,

    有幾百萬種單細胞生物

  • yet there are very few we can see with the naked eye.

    但憑我們肉眼能看到的,極其少數

  • Why is that?

    這是為什麼阿?

  • Why don't we get unicellular elephants,

    為什麼沒有單細胞的大象、

  • or blue whales,

    藍鯨

  • or brown bears?

    或棕熊呢?

  • To find out, we have to peer into a cell's guts.

    我們得仔細端詳細胞內部,才能一探究竟

  • This is where most of the cell's functions occur,

    這裡是大部分細胞功能運作的地方

  • enclosed by a cellular membrane

    外層由細胞膜將其包覆

  • that acts as the doorway into and out of the cell.

    並作為細胞的出入口

  • Any resources the cell needs to consume,

    細胞所需的任何養分

  • or waste products it needs to expel,

    或要排除的廢物

  • first have to pass through this membrane.

    都得先通過這層膜

  • But there's a biological quirk in this setup.

    但這樣的結構,在生物上有個奇怪現象

  • A cell's surface and volume increase at different rates.

    細胞的表面積和體積,是以不同比率增加

  • Cells come in many shapes,

    細胞有很多種形狀

  • but imagining them as cubes will make the math easy to calculate.

    把它們想像成立方體,會比較容易計算

  • A cube has six faces.

    一個立方有六個面

  • These represent the cell membrane, and make up its surface area.

    這些面代表細胞膜,組成細胞表面積

  • A cube measuring one micrometer on each side,

    一個每邊長一微米的立方體

  • that's one millionth of a meter,

    一微米就是百萬分之一公尺

  • would have a total surface area of six square micrometers.

    這樣表面積總共是六平方微米

  • And its volume would be one cubic micrometer.

    而它的體積則是一立方微米

  • This would give us six units of surface area for every single unit of volume,

    所以每單位體積會有六單位的表面積

  • a six to one ratio.

    六比一的比例

  • But things change dramatically if we make the cube ten times bigger,

    但若把立方變成十倍大,情況就會截然不同

  • measuring ten micrometers on each side.

    立方每邊長為十微米

  • This cell would have a surface area of 600 square micrometers

    細胞表面積就會是六百平方微米

  • and a volume of one thousand cubic micrometers,

    體積則是一千立方微米

  • a ratio of only .6 to one.

    比例變成只有 0.6 比 1

  • That's less than one unit of surface area to service each unit of volume.

    每單位體積分不到一單位的表面積

  • As the cube grows, its volume increases much faster than its surface area.

    當立方變大時,體積會比表面積增加得更快

  • The interior would overtake the membrane,

    內部體積很快就會讓細胞膜無法負荷

  • leaving too little surface area for things to quickly move in and out of the cell.

    使表面積過小,物質就不能迅速進出細胞

  • A huge cell would back up with waste and eventually die and disintegrate.

    巨型的細胞會屯滿廢物,最後死亡、崩解

  • There's another plus to having multitudes of smaller cells, too.

    擁有許多小細胞,有其好處

  • It's hardly a tragedy if one gets punctured, infected, or destroyed.

    就算其中一個細胞被刺穿、感染或破壞也無傷大雅

  • Now, there are some exceptionally large cells that have adapted to cheat the system,

    有些超大的細胞,找到方法躲過前述生物機制

  • like the body's longest cell,

    像是人體最長的細胞

  • a neuron that stretches from the base of the spine to the foot.

    是從脊髓基部延伸到腳的神經元

  • To compensate for its length, it's really thin,

    作為其長度的代償,細胞非常細

  • just a few micrometers in diameter.

    直徑只有幾微米

  • Another example can be found in your small intestine,

    另一個例子則在小腸裡

  • where structures called villi fold up into little fingers.

    絨毛會摺疊成像小手指的結構

  • Each villus is made of cells with highly folded membranes

    每個絨毛是由表面高度折疊的細胞所組成

  • that have tiny bumps called microvilli to increase their surface area.

    而細胞上面的突起,叫微絨毛,它能增加表面積

  • But what about single-celled organisms?

    那單細胞生物又是如何呢?

  • Caulerpa taxifolia, a green algae that can reach 30 centimeters long,

    杉葉蕨藻,一種可長達三十公分的藻類

  • is believed to be the largest single-celled organism in the world

    被認為是世界上最大的單細胞生物

  • thanks to its unique biological hacks.

    因為它有獨特的生物結構

  • Its surface area is enhanced with a frond-like structure.

    蕨類葉片般的結構提升它的表面積

  • It uses photosynthesis to assemble its own food molecules

    它利用光合作用,自行合成所需養分

  • and it's coenocytic.

    而且它是多核生物

  • That means it's a single cell with multiple nuclei,

    意思是,它是擁有多細胞核的單細胞生物

  • making it like a multicellular organism but without the divisions between cells.

    它就像多細胞生物,只是沒有細胞分裂

  • Yet even the biggest unicellular organisms have limits,

    就算是最大的單細胞生物也有極限

  • and none grows nearly as large as the elephant, whale, or bear.

    所以沒有單細胞生物可以長到跟大象、鯨魚或熊一樣大

  • But within every big creature are trillions of minuscule cells

    但每個大型生物,都是由數兆個微小的細胞組成

  • perfectly suited in all their tininess

    每個細胞都乖乖地固守崗位

  • to keeping the Earth's giants lumbering along.

    好讓地球上的這些龐然大物能遊走四方

The elephant is a creature of epic proportions,

大象是體積超龐大的生物

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B2 中高級 中文 美國腔 TED-Ed 細胞 表面積 生物 體積 細胞膜

【TED-Ed】最大的單細胞生物是什麼?- 穆裡-甘斯 (【TED-Ed】What is the biggest single-celled organism? - Murry Gans)

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    Sh, Gang (Aaron) 發佈於 2021 年 01 月 14 日
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